This is probably a straightforward question, but I wanted to ask just to be on the safe side. I have a Wifi Module that I'm connecting to an ARM Cortex M4 uC, and it interfaces via UART.

Since I tend to throw on ESD and EMI protection on any lines communicating with the outside world, I figured I would add it to the UART lines. Typically, though, the two UART lines, TXD and RXD are purely inter-board - meaning that the only connection is between the module and the uC. In this case, I typically forgo the rigorous ESD protection and simply use a series resistor.

However, I've also added headers on these lines so I can use a logic analyzer and an FTDI chip for programming. This being the case, should I add additional ESD protection? Or could I add switches to disconnect the lines from the uC while programming the module?

If I should add extra protection, could anyone suggest any devices to do so? I've used a few ICs for SD Card interfaces and USB lines, but I have yet to come across one for UART.

Any input is appreciated.

  • \$\begingroup\$ The same ICs you already use should work fine for a USART. \$\endgroup\$ – Ignacio Vazquez-Abrams May 13 '15 at 22:58
  • \$\begingroup\$ @IgnacioVazquez-Abrams That's good to know. Thanks. \$\endgroup\$ – Mlagma May 13 '15 at 23:01
  • \$\begingroup\$ Maybe some reference to specific parts could make this question more useful for the community. Feel free to answer to your own question, if @IgnacioVazquez-Abrams doesn't want to add a complete answer. At least you should edit your question describing what parts you use for protection. \$\endgroup\$ – Lorenzo Donati -- Codidact.org May 13 '15 at 23:58
  • \$\begingroup\$ @LorenzoDonati I'll add an answer once I finish the schematic detailing the portion of my circuit I posted about. \$\endgroup\$ – Mlagma May 14 '15 at 3:14
  • \$\begingroup\$ I'd say it depends on whether the UART is logic-level or RS-232 level. Simply because logic-level is dealing with chip inputs as opposed to RS232, which is ruggedized for very long cabling runs. Yet, if the chips are bipolar instead of MOS as @Dean explains, it will matter far less. Bottom line is, the chips in question very likely have protection diodes on their pins anyways, and would be "safe" for most human-body-model ESD. This may be one of those scenarios where, if you were that concerned, you actually make a test rig, charge up a probe and zap a pin, then record the result. \$\endgroup\$ – rdtsc Dec 28 '15 at 15:33

ESD protection typically applies to integrated circuits which are made using MOS technology. MOS (NMOS, PMOS, CMOS) chips are based on field effect transistors (FETs).

The gate of FET's is a layer of insulating silicon dioxide and acts as a capacitor. Any static charge in a human being can be transferred to the gate on the FET, charge accumulation occurs, and thus the electric field builds in strength across that thin layer which makes up the gate of the FET. Eventually a breakdown event occurs punching a hole through that insulating gate layer, result in permanent damage to the transistor.

It's an effect which affects FET's and not bipolar junction transistors (PNP, NPN).

So if you've got integrated circuits in the design fabricated from MOS technology, then some kind of protection would be good, although MOS integrated circuits tend to have input protection circuits to protect them, but that's really dealing with how the chips are handled before insertion into a circuit board.

I'm thinking about the source of the ESD, where do you think it can come from? ESD doesn't normally occur. Are you thinking about thunderstorms and lightening which can induce high voltages into equipment? I've seen thunderstorms take out electronic apparatus without a direct strike on the equipment, the flash of lightening inducing a high enough voltage in the electronics so as to destroy components.

Electronics with long cable runs are going to be more susceptible, particularly if exposed, located outside of the equipment. If you have cables like this then you might want to think about opto-isolators. I've even seen these fried by a thunderstorm close by, but at least the main printed circuit board and components were protected and not damaged by the lightening.


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